Diabetes mellitus is a systemic disease characterized by metabolic disorders involving insulin, carbohydrates, fats and proteins, and disorders in the structure and function of blood vessels. The primary symptom of acute diabetes is hyperglycemia, often accompanied by glucosuria, the presence in urine of large amounts of glucose, and polyuria, the excretion of large volumes of urine. Additional symptoms arise in chronic diabetes, including degeneration of the walls of blood vessels. Although many different human organs are affected by these vascular changes, the eyes and kidneys appear to be the most susceptible. As such, long-standing diabetes mellitus, even when treated with insulin, is a leading cause of blindness.
There are three recognized types of diabetes mellitus. Type I diabetes or insulin dependent diabetes mellitus (IDDM) is typically of juvenile onset; ketosis develops early in life with much more severe symptoms and has a near-certain prospect of later vascular involvement. Control of Type I diabetes is difficult and requires exogenous insulin administration. Type II diabetes or non-insulin dependent diabetes mellitus (NIDDM) is ketosis-resistant, generally develops later in life, is milder and has a more gradual onset. Gestational diabetes is related to type II diabetes and associated with an increased risk of later development of that disease. Type III diabetes is malnutrition-related diabetes.
NIDDM is a condition that poses a major threat to the health of the citizens of the western world. NIDDM accounts for over 85% of diabetes incidence worldwide and about 160 million people are suffering from NIDDM. The incidence is expected to increase considerably within the next decades, especially in developing countries. NIDDM is associated with morbidity and premature mortality resulting from serious complications, e.g., cardiovascular disease (G. C. Weir and J. L. Leahy, Pathogenesis of non-insulin dependent (Type II) diabetes mellitus, in Joslin's Diabetes Mellitus (Eds. C. R. Kahn and G. C. Weir), 13th Edition, 1994, Lea & Febiger, Malvern, Pa., pp. 240-264). NIDDM is characterized by both fasting and post-prandial hyperglycemia resulting from abnormalities in insulin secretion and insulin action (G. C. Weir et al., vide supra).
The hyperglycemia in patients suffering from NIDDM can usually be initially treated by dieting, but eventually most NIDDM patients have to take oral antidiabetic agents and/or insulin injections to normalize their blood glucose levels. The introduction of orally effective hypoglycemic agents was an important development in the treatment of hyperglycemia by lowering blood glucose levels. Currently, the most widely used oral antidiabetic agents are the sulfonylureas, which act by increasing the secretion of insulin from the pancreas (H. E. Lebovitz, Oral antidiabetic agents, in Joslin's Diabetes Mellitus (Eds. C. R. Kahn and G. C. Weir), 13th Edition, 1994, Lea & Febiger, Malvern, Pa., pp. 508-529), the biguanides (e.g., metformin) which act on the liver and periphery by unknown mechanisms (C. J. Bailey, M. R. C. Path and R. C. Turner N. Engl. J. Med. 1996, 334, 574-579) and the thiazolidinediones (e.g., rosiglitazone/Avandia®), which enhance the effects of insulin at peripheral target sites (G. L. Plosker and D. Faulds Drugs 1999, 57, 409-438). These existing therapies which comprise a wide variety of biguanide, sulfonylurea and thiazolidinedione derivatives have been used clinically as hypoglycemic agents. However, all three classes of compound have side effects. The biguanides, for example metformin, are unspecific and in certain cases have been associated with lactic acidosis, and need to be given over a longer period of time, i.e. they are not suitable for acute administration (C. J. Bailey et al., vide supra). The sulfonylureas, though having good hypoglycemic activity, require great care during use because they frequently cause serious hypoglycemia and are most effective over a period of circa ten years. The thiazolidinediones may cause weight gain following chronic administration (G. L. Plosker and D. Faulds, vide supra) and troglitazone has been associated with the occurrence of serious hepatic dysfunction.
Thus, there is a significant and rising need for antidiabetic drugs that have novel mechanisms of action, thereby avoiding side effects produced by known therapies. The hormone somatostatin (SST) is primarily produced in the intestinal tract and in the pancreas. In addition it acts as a neurotransmitter. The hormone is involved through its receptors in the regulation of several other hormones and in immunoregulation. In particular, SST suppresses the secretion of insulin by pancreatic β cells and the secretion of glucagon-like peptide 1 (GLP-1) by L cells. GLP-1 in turn is one of the most potent stimulators of insulin production and secretion and is a trophic factor for β cells. β and L cells express SST receptor subtype 5 (SSTR5) and agonizing this receptor suppresses insulin and GLP-1 secretion in humans and in animal models (e.g., Y. Zambre, Z. Ling, M.-C. Chen, X. Hou, C.-W. Woon, M. Culler, J. E. Taylor, D. H. Coy, C. van Schravendijk, F. Schuit, D. G. Pipeleers and D. L. Eizirik Biochem. Pharmacol. 1999, 57, 1159-1164; S. P. Fagan, A. Azizzadeh, S. Moldovan, M. K. Ray, T. E. Adrian, X. Ding, D. H. Coy and F. C. Brunicardi Surgery 1998, 124, 254-258; M. Norman, S. Moldovan, V. Seghers, X.-P. Wang, F. J. DeMayo and F. C. Brunicardi Ann. Surg. 2002, 235, 767-774; T. A. Tirone, M. A. Norman, S. Moldovan, F. J. DeMayo, X.-P. Wang and F. C. Brunicardi Pancreas 2003, 26, e67-73; M. Z. Strowski, M. Köhler, H. Y. Chen, M. E. Trumbauer, Z. Li, D. Szalkowski, S. Gopal-Truter, J. K. Fisher, J. M. Schaeffer, A. D. Blake, B. B. Zhang and H. A. Wilkinson Mol. Endocrinol. 2003, 17, 93-106).
Consequently, antagonizing the effect of SST would lead to higher plasma insulin concentrations. In patients suffering from impaired glucose tolerance and NIDDM, a higher plasma insulin concentration would moderate the dangerous hyperglycemia and accordingly reduce the risk of tissue damage. If such SSTR5 antagonists are sufficiently selective over the other four SST receptors, little influence is expected on secretion of other hormones. Particularly, selectivity over SST receptor subtype 2 avoids influences on glucagon secretion (K. Cejvan, D. H. Coy and S. Efendic Diabetes 2003, 52, 1176-1181; M. Z. Strowski, R. M. Parmar, A. D. Blake and J. M. Schaeffer Endocrinology 2000, 141, 111-117). Advantageous over established therapies is the dual mechanism of action to increase insulin secretion: directly on pancreatic a cells and indirectly through GLP-1 release from L cells. Additionally, SSTR5 knockout mice demonstrated higher insulin sensitivity than littermates (M. Z. Strowski, M. Köhler et al., vide supra). Therefore, SSTR5 antagonists could have the potential to beneficially influence insulin resistance in patients with NIDDM. In summary, SSTR5 antagonists are expected to beneficially influence NIDDM, the underlying impaired fasting glucose and impaired glucose tolerance, as well as complications of long-standing, insufficiently controlled diabetes mellitus.
GLP-1 is known as an endogenous regulator of gastrointestinal motility and of food intake reducing appetite as shown in laboratory animals, healthy volunteers and patients with NIDDM (E. Näslund, B. Barkeling, N. King, M. Gutniak, J. E. Blundell, J. J. Holst, S. Rössner and P. M. Hellström Int. J. Obes. 1999, 23, 304-311; J.-P. Gutzwiller, B. Göke, J. Drewe, P. Hildebrand, S. Ketterer, D. Handschin, R. Winterhalder, D. Conen and C. Beglinger Gut 1999, 44, 81-88; J.-P. Gutzwiller, J. Drewe, B. Göke, H. Schmidt, B. Rohrer, J. Lareida and C. Beglinger Am. J. Physiol. 1999, 276, R1541-1544; M. D. Turton, D. O'Shea, I. Gunn, S. A. Beak, C. M. Edwards, K. Meeran, S. J. Choi, G. M. Taylor, M. M. Heath, P. D. Lambert, J. P. Wilding, D. M. Smith, M. A. Ghatei, J. Herbert and S. R. Bloom Nature 1996, 379, 69-72; A. Flint, A. Raben, A. Astrup and J. J. Holst J. Clin. Invest. 1998, 101, 515-520; M. B. Toft-Nielsen, S. Madsbad and J. J. Holst Diabetes Care 1999, 22, 1137-1143; P. K. Cheikani, A. C. Haver and R. D. Reidelberger Am. J. Physiol. 2005, 288, R1695-R1706; T. Miki, K. Minami, H. Shinozaki, K. Matsumura, A. Saraya, H. Ikeda, Y. Yamada, J. J. Holst and S. Seino Diabetes 2005, 54, 1056-1063); thus, elevated GLP-1 will also counteract obesity, a typical condition associated with and leading to NIDDM.
GLP-1 is co-secreted with GLP-2 that is, consequently, also regulated by SST through SSTR5 (L. Hansen, B. Hartmann, T. Bisgaard, H. Mineo, P. N. Jørgensen and J. J. Holst Am. J. Phys. 2000, 278, E1010-1018). GLP-2 is enterotrophic and beneficial in patients with malabsorption of certain origins, such as short bowel syndrome (D. G. Burrin, B. Stoll and X. Guan Domest. Anim. Endocrinol. 2003, 24, 103-122; K. V. Haderslev, P. B. Jeppesen, B. Hartmann, J. Thulesen, H. A. Sorensen, J. Graff, B. S. Hansen, F. Tofteng, S. S. Poulsen, J. L. Madsen, J. J. Holst, M. Staun and P. B. Mortensen Scand. J. Gastroenterol. 2002, 37, 392-398; P. B. Jeppesen J. Nutr. 2003, 133, 3721-3724).
Moreover, there is increasing evidence for a role of SST on immune cells and expression of SSTR5 on activated T lymphocytes (T. Talme, J. Ivanoff, M. Hägglund, R. J. J. van Neerven, A. Ivanoff and K. G. Sundqvist Clin. Exp. Immunol. 2001, 125, 71-79; D. Ferone, P. M. van Hagen, C. Semino, V. A. Dalm, A. Barreca, A. Colao, S. W. J. Lamberts, F. Minuto and L. J. Hofland Dig. Liver Dis. 2004, 36, S68-77; C. E. Ghamrawy, C. Rabourdin-Combe and S. Krantic Peptides 1999, 20, 305-311). Consequently, SSTR5 antagonists could also prove valuable in treating diseases characterized by a disturbed immune system, such as inflammatory bowel disease.